WDM (Wavelength Division Multiplexing) which provides a high capacity point-to-point connection and a ROADM (Reconfigurable Optical Add Drop Multiplexer) are introduced in an optical communication core network. Further, the WDM and the ROADM are being introduced to a Metro network and a regional network.
In the ROADM system in which the introduced WDM apparatus is used as a base platform, a transponder in a node is required to have a function to connect the transponder with a path by using an arbitrary optical path and an arbitrary wavelength.
As an optical device having a function to select a light signal from a plurality of light signals transmitted through a plurality of paths and output it to the transponder, a multicast switch described in non-patent document 1 is used. The non-patent document 1 describes a multicast switch which can connect four paths with eight transponders.
FIG. 8 is a figure for explaining operation of the multicast switch. In FIG. 8, for ease of explanation, a case in which transponders 804-1 to 804-4 are connected to a multicast switch 805 that can connect four paths with four transponders is shown. The multicast switch 805 described in FIG. 8 differs from the multicast switch described in non-patent document 1 in that up to four transponders can be connected. However, the basic operation of the multicast switch described in non-patent document 1 is the same as that of the multicast switch 805 described in FIG. 8.
In FIG. 8, a wavelength multiplexed light transmitted through each path is inputted to one of 1×4 splitters 801-1 to 801-4. The light inputted to one of the 1×4 splitters 801-1 to 801-4 is divided into four lights. Four divided wavelength multiplexed lights are inputted to 4×1 switches 802-1 to 802-4 whose number is equal to the number of the transponders, respectively. In FIG. 8, the number of the transponders connected to the multicast switch 805 is four. Therefore, four 4×1 switches are used.
The 4×1 switches 802-1 to 802-4 select one of the wavelength multiplexed lights inputted from the 1×4 splitters 801-1 to 801-4 and output the selected light to wavelength variable filters 803-1 to 803-4, respectively. The transmission wavelength of the wavelength variable filters 803-1 to 803-4 is set so that it is equal to the wavelength of the light to be received by the connected transponder. As a result, each of the transponders 804-1 to 804-4 receives only the light having a target wavelength that is extracted from a plurality of the lights included in the wavelength multiplexed light by the wavelength variable filters 803-1 to 803-4.
Here, the number of the 4×1 splitters 801-1 to 801-4 is made equal to the number of the paths. Each of the 4×1 optical switches selects the path that is used for communication by the transponder to which the 4×1 optical switches are connected.
The reception operation of the transponder has been described above by using FIG. 8. When the transponders 804-1 to 804-4 transmit the light, the 4×1 optical switches 802-1 to 802-4 switch the optical path so that the lights transmitted by the transponders 804-1 to 804-4 are transmitted to the splitters 801-1 to 801-4 connected to the predetermined optical path. As a result, the output of the transponder is outputted to the arbitrary optical path. This operation is the same as that of the multicast switch described in non-patent document 1 to which eight transponders can be connected.
As mentioned above, the multicast switch shown in FIG. 8 or non-patent document 1 can set the path to send the light transmitted through the arbitrary path to the arbitrary transponder and additionally, the same wavelength can be used in a plurality of optical paths.
Patent document 1 and patent document 2 describe structures in which the switching is performed by the optical switch while making the wavelength of the optical transmitter and receiver correspond to the wavelength of the transmission path.